1. Field of the Invention
The present invention relates to an intermediate system connected to a packet-switched real time communication network and for relaying communication data transmitted from a single transmitting terminal and for distributing the communication data to a plurality of receiving terminals and a method of controlling a relay. More specifically, the present invention relates to an intermediate system for dynamically controlling an amount of data per unit time of the communication data to be distributed to each receiving terminal in response to a congestion in each receiving terminal and a method of controlling a relay.
2. Description of the Related Art
A demand for transmitting/receiving real time information such as a television conference via a network has been recently increased. As a protocol for transmitting a sound, an image or the like at a real time on a TCP/IP (Transmission Control Protocol/Internet Protocol) network, attention is paid to an RTP (Real time Transport Protocol) and an RTCP (RTP Control Protocol).
In case of the television conference or the like, the connection thereof is accomplished in the form of a point (a single transmitting terminal) to-multi-point (a plurality of receiving terminals), that is, a multi-cast communication. However, in an environment in which various networks are interconnected to one another, all users cannot accomplish a communication in the same environment. Most of connection networks are a resource shared LAN which shares resource locally. Thus, the networks do not ensure that they can always use a constant network resource.
A plurality of receiving terminals sometimes include the receiving terminal connected to the network via a relatively low-speed communication circuit such as a public telephone circuit and the receiving terminal connected to the network via a relatively high-speed communication circuit such as Ethernet. However, in the conventional multi-cast communication, the transmitting terminal cannot control such that a transmission speed of communication data is changed for each receiving terminal. Therefore, when a congestion is caused in the network and the receiving terminal, even if the congestion is partially generated, the transmitting terminal performs a congestion control for reducing an amount of data to be transmitted. Thus, the amount of data to be transmitted to other receiving terminals, in which the congestion is not caused, is also reduced in a uniform manner. This causes a deterioration of communication quality. Disadvantageously, this causes an unnecessary degradation of the sound and an image quality.
In order to solve such a problem, in the RTP, an intermediate system (mixer) for making the communication in various environments possible is defined. The conventional mixer is provided with a function for decoding/re-encoding and relaying the received data. When the communication data transmitted from the transmitting terminal is decoded/re-encoded, for the high-communicability receiving terminal, the received communication data is not reduced but processed and distributed to the receiving terminal. On the other hand, for the low-communicability receiving terminal, the communication data transmitted from the transmitting terminal is partially discarded and the amount of the communication data to be distributed is reduced, whereby the congestion and a packet loss is prevented.
As described above, a real time communication requires the congestion control in order not to deteriorate the congestion in the network and a rate of packet loss. In the prior art using the above-described mixer, the congestion control is only statically performed in accordance with a physical capability such as the capability of the receiving terminal itself or the network itself. A dynamic control is not performed in accordance with the actual congestion, the actual packet loss or the like.
However, the congestion, the packet loss or the like is dynamically varied not only in response to the physical capability of the receiving terminal or the like but also in response to a load of CPU mounted in each receiving terminal, a use condition of the network or the like. Therefore, it is difficult for a static congestion control alone based on the physical capability of the receiving terminal or the like to effectively prevent the congestion, the packet loss or the like from occurring and deteriorating.